The present invention relates to an electronic blood pressure meter.
An electronic blood pressure meter generally determines systolic and diastolic pressures by first inflating a cuff to pressurize an artery of a human body and then using information on the blood vessel such as pulse wave and K-sounds as well as cuff pressure in a subsequent deflation process.
There have conventionally been some deflation methods namely deflation methods as discussed below, employed in this type of electronic blood pressure meter in measurement.
1. Displacement adjusting means is controlled to allow pressure to decrease at a constant rate which is set in advance.
2. Displacement adjusting means is controlled to allow pressure to decrease at a rate calculated according to a detected pressure in a cuff and pulse (Japanese Patent Laying-Open Nos. 5-84221 and 5-337090).
3. Deflation rate is adjusted according to irregularities in the amplitude of pulse wave in inflation (Japanese Patent Laying-Open No. 5-200005).
A conventional electronic blood pressure meter has a system of automatically setting of inflate pressure by estimating from pulse wave observed in inflation a systolic pressure or any relevant value and adding a constant pressure to the estimated systolic pressure to use the resultant value for setting inflation.
A conventional electronic blood pressure meter employs a system of controlling a deflation rate. This system implements a high speed control by changing a voltage applied to a control valve at short periods to enable the deflation rate to speedily reach a target rate after inflation is completed and then using normal control periods when the deflation rate attains the target rate in order to calculate blood pressure.
One of those systems of deflation employed in conventional electronic blood pressure meters, the first system implements a constant deflation rate. If a subject has a low pulse or a low pulse pressure, the number of pulse waves detected during displacement between the systolic and diastolic pressures is small, and accordingly an accurate measurement of blood pressure is impossible. When the displacement rate is decreased for detecting a sufficient number of pulse waves, a longer time is required for measurement, resulting in a greater load on the subject due to congestion and long-term inflation.
According to the second system, the deflation rate is independent of pulse pressure. Then, the number of pulse waves of a subject having a low pulse pressure is small, which deteriorates measurement precision.
On the other hand, a subject having hypertension and a high pulse pressure feels uncomfortable and pain, because of inflation by a cuff for an excessively long period of time.
The third system has a problem that the deflation rate cannot be increased when a subject has a normal pulse.
The automatic inflation setting system of the conventional electronic blood pressure meter discussed above always adds a fixed value to a systolic pressure. Therefore, particularly a subject having a relatively fast pulse is excessively pressurized in measurement, feeling uncomfortable and pain.
In order to set the deflation rate higher than a conventional rate, a fixed value (inflation margin) to be added must be large since it is necessary to detect at least one pulsation at a pressure higher than the systolic pressure in deflation.
The system of controlling deflation rate employed in the conventional electronic blood pressure meter sets a high initial value as a control valve drive voltage that is enough to close the valve. Therefore, even if a high-rate control is conducted, there occurs a delay of a few seconds before a drive voltage is reached that enables the target deflation rate. Resultant problems are that measurement takes a longer time and that measurement cannot be started until pressure is applied to a human body more than necessary.
The present invention has been made to overcome the problems above. One object of the invention is to provide an electronic blood pressure meter capable of taking measurements as fast as possible without deteriorating precision.
An electronic blood pressure meter according to the present invention determines blood pressure in a process of gradually depressurizing an artery of at least one area of a human body that has been pressurized to a level higher than systolic pressure. The electronic blood pressure meter includes pulse wave detecting means for detecting oscillation of a blood vessel wall or pulsating blood flow in the artery occurring in the inflation process, pulse wave period measuring means calculating, when the pulse wave detecting means detects a plurality of pulse waves, a period between occurrences of respective pulsations, pulse wave feature amount calculating means receiving as inputs a pulse wave signal of one or a plurality of pulsations supplied from the pulse wave detecting means and a representative pressure value synchronizing with pulse wave generation timing supplied from pressure detecting means for detecting pressure at the pressurized area of the human body, for extracting a feature amount correlated to at least two of systolic pressure, diastolic pressure and mean blood pressure or correlated to systolic pressure only, and deflation rate calculating means according to an output from the pulse wave feature amount calculating means and the pulse wave period measured by the pulse wave period measuring means for calculating a target deflation rate. The deflation rate calculating means uses the feature amount calculated by the pulse wave feature amount calculating means to calculate and determine the target deflation rate to enable detection of a predetermined number of pulse waves between systolic pressure and diastolic pressure in measurement performed in the deflation process.
An optimum deflation rate is thus determined and accordingly pulse waves that can be detected in deflation are minimum in number while the number satisfies a number necessary for assuring precision.
Preferably, the electronic blood pressure meter further includes target inflation value setting means for determining a target inflation value according to an output from the pulse wave feature amount calculating means.
An optimum and minimum target inflation value is thus determined and accordingly only a minimum pressure that is necessary is applied to avoid excessive inflation by extra pressure.
Further, an electronic blood pressure meter according to the invention determines blood pressure in a process of gradually depressurizing an artery of at least one area of a human body that has been pressurized to a level higher than systolic pressure, and includes fluid pressure control means for controlling deflation rate by adjusting flow rate of fluid. The electronic blood pressure meter further includes means for detecting information on change of inflation force in the inflation process, and initial rate-control-value determining means using the pressure change information for indicating an initial value of a feature amount at start of deflation to the fluid pressure control means. Based on the pressure rising state in the inflation process, pressure pump drive voltage, pressure at start of deflation and voltage-flow rate characteristics of a control valve, a voltage is applied as an initial value to drive the control valve such that a flow rate is accomplished to immediately reach a target rate. Then, immediately after inflation is completed, deflation is stably controlled at a target deflation rate. There is no wasteful time nor extra inflation before control becomes stable, and thus the time and pressure are minimum that are required while precision is assured.